1. Field of the Invention
The present invention relates to a support structure of a motor-driven steering assist apparatus.
2. Description of the Invention
In a rough road traveling vehicle such as a buggy vehicle or the like, as described in Japanese Patent Application Laid-open No. 2004-231011 (patent document 1), there is a structure in which a motor-driven steering assist apparatus is interposed between an upper steering shaft in a steering wheel side and a lower steering shaft in a tire wheel side. A steering force applied to the steering wheel by a driver is assisted by a torque generated by an electric motor.
In the motor-driven steering assist apparatus in the patent document 1, a single unit body covered by a housing has an input shaft to which the upper steering shaft is connected, and an output shaft to which the lower steering shaft is connected. A torque sensor is provided between the input shaft and the output shaft. An electric motor is driven in correspondence to a detected torque of the torque sensor. A worm gear is coupled to a rotary shaft of the electric motor, and a worm wheel is coupled to the output shaft and engaging with the worm gear built-in. The housing can be attached to a vehicle body frame (a vehicle body side bracket). When the steering torque is applied to the steering wheel, the steering torque is applied to a torsion bar coupling the input shaft and the output shaft via the upper steering shaft, and the input shaft and the output shaft relatively rotate on the basis of a torsion generated by the torsion bar. Accordingly, the torque sensor detects the steering torque on the basis of a displacement in a rotating direction of the input shaft and the output shaft, and the electric motor is controlled so as to apply an assist force in correspondence to the steering torque to the output shaft.
In the motor-driven steering assist apparatus provided in the conventional rough road traveling vehicle, when steering the steering wheel to the maximum, the upper steering shaft and the input shaft twists the torsion bar, so that the torque sensor carries on detecting the steering torque. Accordingly, the electric motor is continuously driven despite that the output shaft and the lower steering shaft reach a steering stroke end.
Further, in the motor-driven steering assist apparatus provided in the conventional rough road traveling vehicle, if an inverse input is continuously input from the tire wheel side due to a rock being caught on the tire wheel during steering the steering wheel to the maximum, the lower steering shaft and the output shaft continues twisting the torsion bar, so that the torque sensor continues to detect the steering torque. Accordingly, the electric motor is continuously driven despite that the output shaft and the lower steering shaft reach the steering stroke end, thereby experiencing breakage such as a burnout or the like.